1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and particularly to a spread illuminating apparatus used as an illuminating means for a liquid crystal display.
2. Description of the Related Art
A liquid crystal display characterized by its low profile, small occupied volume, light weight, etc. has been extensively used in electronic products including portable telephones and notebook computers and the demand thereof has been increased. However, since a liquid crystal of the liquid crystal display does not emit light by itself, a separate illuminating means is needed besides the liquid crystal display when used in dark places where solar or room light is not satisfactorily available. Thus, this illuminating means is required to be compact in size and small in power consumption. In recent years, a thin plate-like spread illuminating apparatus of side light type has often been applied.
FIG. 6 shows an embodiment of a conventional spread illuminating apparatus of side light type.
As shown in the figure, a spread illuminating apparatus 1xe2x80x2 is generally composed of a transparent substrate 2 made of a light-transmissible material and a bar-like light source 5 disposed close to an end surface 8 of the transparent substrate 2. Light rays emitted from the light source 5 are introduced into the transparent substrate 2 to illuminate a liquid crystal display (not shown in the figure) disposed under the transparent substrate 2.
The light source 5 is composed of a bar-like light conductive member 3 made of a transparent material and a spot-like light source (for example, a light-emitting diode) 4 disposed facing one end 7 of the light conductive member 3. The light conductive member 3 is provided with an optical path conversion means 12. The optical path conversion means 12 includes grooves, for example, triangular in cross section, located on a surface opposite to a surface (one side surface) 9 of the bar-like light conductive member 3 facing the end surface 8 of the transparent substrate 2, and thereby functions to allow light rays emitted from the spot-like light source 4 to be made incident on the end surface 8 of the transparent substrate 2 in a substantially uniform manner. The light conductive member 3 is disposed with the surface 9 running along the end surface 8 of the transparent substrate 2 at a predetermined distance therefrom.
A light reflection member (frame) 13 is provided around the light conductive member 3 so as to efficiently guide the light rays emitted from the spot-like light source 4 into the transparent substrate 2. The light reflection member 13 is substantially U-shaped and covers longitudinal surfaces of the light conductive member except the surface 9 facing the transparent substrate 2.
A light reflection pattern 39 is formed on an upper surface 15 of the transparent substrate 2. The light reflection pattern 39 comprises grooves 37 triangular in section and flat portions 38 adjacent thereto, both formed parallel to the light conductive member 3 with predetermined intervals between each other. The depth of the grooves 37 increases gradually in proportion to the increase in distance from the light conductive member 3 so that light coming from the light conductive member 3 and entering the transparent substrate 2 is reflected at a whole surface of the transparent substrate 2 in a substantially uniform manner irrespective of the distance from the light conductive member 3 (the light source 5) thereby uniformly illuminating a liquid crystal display (not shown in the figure) provided under the transparent substrate 2.
In addition, the light reflection pattern 39 may consist of prisms forming a stair-like configuration.
However, in a front light system in which a spread illuminating apparatus using such a light reflection pattern 39 is placed on a front face of a reflection-type liquid crystal display element, interference fringes (moirxc3xa9 pattern) created by fringes formed due to the difference in transmissivity (difference in a transmitting direction due to refraction) between the grooves 37 and the flat portions 38 and the array of mosaic pattern (cell boundary) of liquid crystal cells constituting pixels of the liquid crystal display (liquid crystal display element) are noticeable when light rays reflected at the reflection-type liquid crystal element pass through the light reflection pattern 39, which causes difficulty in observing an image. The appearance of the moirxc3xa9 pattern is closely related to the pattern shape of the light reflection pattern 39, i.e., the depth, size and interval of the grooves 37. Since the moirxc3xa9 pattern is a phenomenon unique to the front light system, there is a disadvantage that it appears at different positions and intervals according to a change of an observer""s visual angle and that there is an appearance of light-and-dark fringes which are different in type from moirxc3xa9 fringes.
In view of the foregoing matters, the inventors of the present invention disclosed an invention in the Japanese Patent Application Laid-Open No. Hei 11-260128 as a countermeasure for the moirxc3xa9 fringes, featuring that a predetermined relation is satisfied between the interval of the grooves of the light reflection pattern formed on the transparent substrate and the interval of the pattern arrayed parallel to the grooves on the transparent substrate out of the mosaic patterns arrayed of the liquid crystal cells of the reflection-type liquid crystal display element.
Furthermore, the Japanese Patent Application Laid-Open No. Hei 11-339526 discloses an invention in which, a light diffusion plate is disposed over the reflection-type liquid crystal display element to diffuse light rays emitted from the transparent substrate, thus preventing the light-and-dark fringes from appearing on the upper surface of the transparent substrate, whereby the appearance of the interference fringes (moirxc3xa9 pattern) due to light-and-dark fringes and the array of the liquid crystal cells of the reflection-type liquid crystal display element is suppressed.
Moreover, the monthly publication xe2x80x9cDISPLAYxe2x80x9d (April 2000 Issue) discloses a method for optimizing the elimination of moirxc3xa9 fringes by providing a predetermined angle xcex8 (xcex8=22.5xc2x0) of inclination to a prism ridge and an electrode array. FIG. 7 shows a spread illuminating apparatus 1xe2x80x3 based on this art.
As shown in the figure, a light reflection pattern 69 formed on the upper face of the transparent substrate 2 is composed of grooves 67 and flat portions 68 adjacent thereto. The grooves 67 and the flat portions 68 are formed with a predetermined angle xcex8 of inclination (preferably, xcex8=22.5xc2x0) with respect to the optical axis of the bar-like light source 5.
The spread illuminating apparatus disclosed in the aforementioned Japanese Patent Application Laid-Open No. Hei 11-339526 has been effective as a remedy for moirxc3xa9 pattern but has the following disadvantages.
In the spread illuminating apparatus disclosed in the Japanese Patent Application Laid-Open No. Hei 11-339526, additional parts such as a light diffusion plate for diffusing light must be provided to prevent the light-and-dark fringes from appearing on the upper surface of the transparent substrate, whereby the cost is inevitably increased. Since light is absorbed and diffused when the light passes through the light diffusion plate, the visibility and contrasts are lowered.
Moreover, there has been no effective remedy for the appearance of the light-and-dark fringes positions, the intervals of which vary according to the change of an observer""s visual angle, as causes of their appearance have been unknown.
In the spread illuminating apparatus 1xe2x80x3 shown in FIG. 7, since the grooves 67 are formed with the angle xcex8 of inclination as shown in the figure, an increased amount of light reflected at the grooves is made incident in an inclined manner on the liquid crystal display element side in comparison with a spread illuminating apparatus in which grooves are formed parallel to the optical axis of a light source without an inclination angle, whereby the efficiency of light utilization becomes poor, lowering the brightness.
The present invention has been made in the light of the above problems and the object of the present invention is to provide a spread illuminating apparatus capable of suppressing the appearance of moirxc3xa9 fringes and even light-and-dark fringes, different in type from moirxc3xa9 fringes, at a low cost without providing additional pads, all while maintaining the efficiency of light utilization.
In order to solve the above problems, according to a first aspect of the present invention, in a spread illuminating apparatus of side light type, comprising a transparent substrate made of a light-transmissible material and a longitudinal light source disposed close to a side surface of the transparent substrate, and in which a light reflection pattern consisting of a plurality of grooves and a plurality of flat portions adjacent to the grooves or a light reflection pattern consisting of prisms forming a stair-like configuration, is formed on the transparent substrate, the interval of the grooves of the light reflection pattern is set to be irregular.
Moreover, according to a second aspect of the present invention, in the first aspect, the light reflection pattern is so formed as to be parallel to the optical axis of the light source or to have a fixed angle.
Furthermore, according to a third aspect of the present invention, in the first or second aspect, an ith groove or step from the light source side of the light reflection pattern is formed to be positioned to satisfy the formula
Pi=ixc3x97C+{Bxc3x97(xe2x88x92A+Q/10)}, (i=1, 2, 3 . . . ) 
where Q is a random number, Pi is a distance from the light source side of the light reflection pattern to a place where the ith groove is positioned, C is an average interval between adjacent grooves, B is a variation adjusting value, and A is a deviation adjusting value.
Still further, according to a fourth aspect of the present invention, in any one of the first to third aspects, the variation ratio of each interval of the grooves of the light reflection pattern set to be irregular is calculated by the formula:
[PH={(Pmaxxe2x88x92Ptrg)/Ptrg}xc3x97100], 
or 
[PH={(Ptrgxe2x88x92Pmin)/Ptrg}xc3x97100], 
where PH is the variation ratio, Pmax is the maximum interval value, Pmin is the minimum interval value, and Ptrg (≈C) is the average interval value; and this variation ratio is set to range from 25% to 85%.
Still further, according to a fifth aspect of the present invention, in any one of the first to fourth aspects, each groove is formed to meander.